Pound-force
The pound or pound force (symbol: lb, lbf, lbf, ) is a unit of force in some systems of measurement including English engineering units and British gravitational units.The Engineering ToolBox Definitions The pound-force is equal to the gravitational force exerted on a mass of one avoirdupois pound on the surface of Earth. Since the 18th century, the unit has been used in low-precision measurements, for which small changes in Earth's gravity (which varies from place to place by up to half a percent) can safely be neglected.Acceleration due to gravity varies over the surface of the Earth, generally increasing from about 9.78 m/s2 (32.1 ft/s2) at the equator to about 9.83 m/s2 (32.3 ft/s2) at the poles. The 20th century, however, brought the need for a more precise definition. A standardized value for acceleration due to gravity was therefore needed. Today, in accordance with the , standard gravity is usually taken to be 9.80665 m/s2 (32.174 049 ft/s2). In 1901 the third CGPM declared (second resolution) that: The value adopted in the International Service of Weights and Measures for the standard acceleration due to Earth's gravity is 980.665 cm/s2, value already stated in the laws of some countries. This value was the conventional reference for calculating the kilogram-force, a unit of force whose use has been deprecated since the introduction of SI.Barry N. Taylor, Guide for the Use of the International System of Units (SI), 1995, Special Publication 811, Appendix B note 24 The acceleration of the standard gravitational field (g''n'') and the international avoirdupois pound (lb''m'') define the pound-force as:The international avoirdupois pound is defined to be exactly 0.45359237 kg. : \begin{align} 1\,\mathrm{lb}_f &= 1\,\mathrm{lb}_m \cdot g_n \\ &= 1\,\mathrm{lb}_m \cdot 32.174049\,\mathrm{\tfrac{ft}{s^2}}\\ &= 32.174049\,\mathrm{\tfrac{ft\cdot lb_m}{s^2}}\end{align} : \begin{align} or\end{align} : \begin{align} 1\,\mathrm{lb}_f &= 0.45359237\,\mathrm{kg} \cdot 9.80665\,\mathrm{\tfrac{m}{s^2}}\\ &= 4.4482216152605\,\mathrm{N} (\mbox{exact})\end{align} This definition can be rephrased in terms of the slug. A slug has a mass unit equivalent of 32.174049 lb''m''. A pound-force is the amount of force required to accelerate a slug at a rate of 1 ft/s2, so: : 1\,\mathrm{lb}_f = 1\,\mathrm{slug} \cdot 1\,\mathrm{\tfrac{ft}{s^2}} ::: = 1\,\mathrm{slug}\,\mathrm{\tfrac{ft}{s^2}} Conversion to other units Foot-pound-second systems of units In some contexts, the term "pound" is used almost exclusively to refer to the unit of force and not the unit of mass. In those applications, the preferred unit of mass is the slug, i.e. lb''f''·s2/ft. In other contexts, the unit "pound" refers to a unit of mass. In circumstances where there may otherwise be ambiguity, the symbols "lb''f''" and "lb''m''" and the terms "pounds-force" and "pounds-mass" can be used to distinguish. In the gravitational systems, the weight of the mass unit (pound-mass) on Earth's surface is approximately equal to the force unit (pound-force). This is convenient because one pound mass exerts one pound force due to gravity. Note, however, unlike the other systems the force unit is not equal to the mass unit multiplied by the acceleration unitThe acceleration unit is the distance unit divided by the time unit squared.—the use of Newton's Second Law, F = m·a, requires another factor, gc, usually taken to be 32.174049 lbm·ft/lbf·s2 = 32.174049 lbm/slug. "Absolute" systems are coherent systems of units: by using the slug as the unit of mass, the "engineering" FPS system avoids the need for such a constant. The SI is an "absolute" metric system with kilogram and meter as base units. See also * Poundal * Foot-pound (energy) * Pound-mass * Force * mass * Mass versus weight for the difference between the two physical properties * Weight for a more complete discussion of customary units of force and mass * Pounds per square inch, a unit of pressure * * * * Notes and references * Obert, Edward F., “THERMODYNAMICS”, D.J. Leggett Book Company Inc., New York 1948; Chapter I, Survey of Dimensions and Units, pages 1-24. Category:Units of force or weight Category:Imperial units Category:United States customary units of mass or weight